A splitter circuit is widely used, such as in an STB (set-top box)/TV for CATV/digital TV. A splitter circuit including an active device, such as FET, that is, active splitter circuit, is used in case a gain is needed. Among the important properties of the splitter circuit is isolation among its multiple output ports. Since a feedback circuit from an output to an input is usually provided to increase the frequency bandwidth, sufficient performance cannot be achieved by simply improving the isolation of the FET as the active device. It is necessary to improve the isolation in a peripheral circuit of the FET.
FIG. 9 depicts a circuit diagram of a splitter circuit disclosed in Patent Document 1. An input terminal 116 is grounded via a capacitive device C, while being connected to the gates of FETs Q1, Q2 via an inductor L1. The FET Q1 has a source grounded, and has a drain connected to an output port 118. A feedback circuit (impedance) 121, made up of an inductor L2 and a resistor R1, is connected between the drain and the gate of the FET Q1. The FET Q2 has a source grounded and has a drain connected to an output port 120. A feedback circuit (impedance) 123, made up of an inductor L3 and a resistor R2, is connected between the drain and the gate of the FET Q2. To improve the isolation between the output ports 118 and 120, a (interport impedance) circuit 114, made up of a resistor R3 and an inductor L4, is connected between the output ports 118 and 120.
With the above-described splitter circuit, when the FETs Q1 and Q2 are in operation, an RF signal, fed from the output port 118, is branched into a signal passing through the circuit 114 and a signal passing through the feedback circuit 121 of the FET Q1 or through the FET Q1. The signal that has passed through the circuit 121 or through the FET Q1 further passes through signal paths 110, 112 and is delivered to the FET Q2 where it is amplified. At this time, the signal undergoes phase shifting by 180 degrees and is delivered as an output to the output port 120. On the other hand, the signal that has passed through the circuit 114 gets to the output port 120 without undergoing phase shifting. Hence, the signal that has its phase shifted and the signal that has its phase not shifted will cancel each other, at the output port 120, with a result that the signal at the output port 120 is attenuated, thus improving the isolation.
Patent Document 2 also discloses a circuit that operates in a similar manner.    [Patent Document 1]
U.S. Pat. No. 5,045,822    [Patent Document 2]
U.S. Pat. No. 6,577,198